This invention relates to a two-layer or a multi-layer winding for electrical machines, which winding consists of coils comprising conductor bars which have active portions, connected together by end portions. The invention relates more particularly to such a winding in which the conductor bars comprise mutually twisted and individually insulated subconductors, the twist being continued over more than one consecutive conductor bar and the subconductor insulation being continuous over the end portions.
Alternating current-carrying windings for electrical machines of high current loading are commonly built of specially designed conductor bars. These bars comprise a number of subconductors which are mutually insulated and are arranged so as to pass at least once through every level in the bar cross-section along the active length. Due to the fine subdivision of the conductors, skin effect losses can be considerably reduced in comparison to an equally sized solid conductor bar. A consequence of each subconductor occupying all levels in the conductor bar is that each subconductor is linked by nearly the same flux, which practically eliminates internal circulating currents amongst the different subconductors.
A conductor bar known as the Roebel bar has proved to be technically the most advantageous, and among all known types of compensated conductor bars it has won the widest acceptance. Such a bar is disclosed for example in the German Pat. No. 277012. This conductor bar consists of two adjacent stacks of flat subconductors twisted together over the active length of the machine. Each subconductor executes a 360.degree. total twist at uniform pitch over the active length so that each subconductor in the bar is wrapped once uniformly about all of the others.
In electrical machines of comparatively short axial length, it is not always possible to attain a full 360.degree. of twist within the active length of the bar. To reduce the number of crossovers in a twisted bar it has been proposed, e.g. in the German Pat. No. 311868, to distribute the total 360.degree. twist over the active portions of two conductor bars, with the subconductor insulation maintained over the interconnecting end portion. In the case of a one-layer winding this arrangement achieves complete suppression of the parasitic currents which flow amongst the individual subconductors and which are referred to as circulating currents. The statement, in the above mentioned patent 311868, that complete compensation of circulating currents is also achieved in this way for a two-layer winding is only true if the transverse leakage flux density distribution over the radial height of the two layers is linear which is not necessarily the case.